1. Field of the Invention
The present invention relates to a molding die used for molding a reinforcement resin coating of an optical fiber junction, a method of reinforcing the optical fiber junction using the same, and an optical fiber cable accommodating an optical fiber having this optical fiber junction.
2. Related Background Art
As an optical fiber accommodated in a submarine optical fiber cable and the like, continuous one having a long length is necessary. When a long-length continuous optical fiber is needed, optical fibers shorter than that are connected together to form it. The optical fibers are usually connected as follows. FIGS. 11A and 11B are perspective views for explaining a junction between optical fibers. FIG. 11A shows a state where glass optical fibers in optical fibers are fusion-spliced, whereas FIG. 11B shows a state where the junction is provided with a reinforcement resin coating. In FIGS. 11A and 11B, 121, 122, 123, and 124 indicate a optical fiber (a coated optical fiber), a glass optical fiber (a bare glass fiber without its coating is removed), fusion-splicing position, and reinforcement resin coating, respectively.
First, as shown in FIG. 1A, at end parts of two optical fibers 121, the coating of optical fibers 121 is removed so as to expose the glass optical fibers 122 in the optical fibers 121, and the end faces of two glass optical fibers 122 are butted against each other so as to carry out fusion splicing. The glass optical fiber exposing portions of two optical fibers have substantially the same length, so that the fusion-splicing position is located substantially at the center of the combined glass optical fiber exposing portion. Then, as shown in FIG. 11B, resin molding is effected on the exposing portion of the glass optical fibers 122, so as to form the reinforcement resin coating 124. The outer diameter of the reinforcement resin coating 124 is substantially the same as that of the optical fiber 121, so that the size of the junction does not hinder the optical fibers 121 from being accommodated in an optical fiber cable in particular.
When the coating of the optical fibers 121 is formed from a UV-curable resin, the reinforcement resin coating 124 is usually formed by a cured UV-curable resin as well. The reinforcement resin coating 124 is formed as follows. FIGS. 12A to 12F are views for explaining a molding die used for forming a reinforcement resin coating and a molding method. FIG. 12A is a plan view of the upper die as seen from its parting face side, whereas FIG. 12B is a side view of the upper die. FIG. 12C is a plan view of the lower die as seen from its parting face side, whereas FIG. 12D is a side view of the lower die. FIG. 12E is a front view showing the molding state, whereas FIG. 12F is a side view thereof.
In FIGS. 12A to 12F, 125, 125a, 125b, 126a and 126b, 126c and 126d, 127, 128, 129, 130, 131, 132, and 133 indicate the molding die, upper die, lower die, parting faces, grooves, cavity, resin injection gate, runner, sprue, optical fiber, glass optical fiber, and ultraviolet light, respectively.
The molding die 125 shown in FIGS. 12A to 12F is constituted by the upper die 125a and the lower die 125b. As shown in FIGS. 12A and 12B, along the parting face 126b, the upper die 125a is formed with the linear groove 126c having a semicircular lateral cross section, and has the resin injection gate 128 communicating with the groove 126c and the runner 129 and sprue 130 linked to the resin injection gate 128. As shown in FIGS. 12C and 12D, along the parting face 126b, the lower die 125b is formed with the linear groove 126d having a semicircular lateral cross section. The groove 126c of the upper die 125a and the groove 126d of the lower die 125b are aligned with each other such that the grooves 126c and 126d constitute the cavity 127 having a circular lateral cross section when they are clamped with their parting faces 126a, 126b opposing each other. The inner diameter of the grooves 126c, 126d, i.e., the inner diameter of the cavity 127 is about 250 μm in general.
The upper die 125a and lower die 125b of the molding die 125 are made of silica glass so as to transmit the ultraviolet light therethrough. In each of FIGS. 12A to 12D, the state where the optical fiber 131 having an exposing portion of the glass optical fiber 132 is inserted is depicted with imaginary lines as well. The inserted optical fiber 131 usually has an outer diameter of about 245 μm, whereas the glass optical fiber 132 has an outer diameter of about 125 μm.
In the following manner, the reinforcement resin coating is formed at the junction between optical fibers 131 by using the molding die 125 shown in FIGS. 12A to 12D. At end parts of two optical fibers 131, the coating is removed, so as to expose the glass optical fibers 132, and the end faces of the glass optical fibers 132 are fusion-spliced to each other. Thus combined glass optical fiber 132 is inserted into the cavity 127 formed by the grooves 126c, 126d, and the upper die 125a and lower die 125b are clamped together. Subsequently, a UV-curable resin is injected from the sprue 130 through the runner 129 and resin injection gate 128 into a void surrounding the glass optical fiber 132 within the cavity 127, so that the void is filled therewith. Then, the ultraviolet light 133 is emitted toward the lower die 125b from therebelow, and the ultraviolet light 133 transmitted through the lower die 125b cures the injected UV-curable resin.
Meanwhile, the resin having entered the void surrounding the glass optical fiber 132 within the cavity 127 from the resin injection gate 128 flows toward both sides of the longitudinal direction within the cavity 127, whereby the void is filled therewith to the vicinity of the coating of the optical fiber 131. At this time, there is a slight gap between the surface of the optical fiber 131 and the inner wall faces of the grooves 126c, 126d since the outer diameter of the optical fiber 131 is about 245 μm, whereas the inner diameter of the grooves 126c, 126d is about 250 μm. Usually, the air pushed away to the vicinity of the coating of the optical fiber 131 due to the injection of the UV-curable resin is expelled to the outside through the above-mentioned gap, whereby the void is completely filled with the UV-curable resin to the vicinity of the optical fiber 131. Though a part of the UV-curable resin reaches the gap between the coating of the optical fiber 131 and the grooves 126c, 126d, this gap is very small, whereas the resin has a viscosity, so that the resin extends along the coating of the optical fiber 131 and does not protrude to the outside of the optical fiber 131, whereby the resin intrudes into the gap by only several millimeters at most.
If the gap between the optical fiber 131 and the inner wall faces of the grooves 126c, 126d is narrowed due to the fluctuation in the outer diameter of the optical fiber 131 and the like, however, the air may not completely be expelled from the gap. In this case, the air may remain near the coating of the optical fiber 131 on both sides thereof, whereby bubbles may occur in the vicinity of both end parts of the reinforcement resin coating. While the UV-curable resin is injected into the cavity 127 from a resin supply apparatus, which is not depicted, by way of the sprue 130, runner 129, and resin injection gate 128, the air may be caught at the front end portion of the resin flow in the course of injection, whereby the resin at the front end portion may attain a state including bubbles. Since the vicinity of ends of the coating is filled with the front end portion of the resin flow, bubbles are likely to occur in the vicinity of ends of the coating not only due to the air remaining without being expelled but also due to the air caught into the resin in the course of resin flow.
When an optical fiber is used as being accommodated in a submarine optical fiber cable, the optical fiber receives a large lateral pressure. If bubbles exist within the reinforcement resin coating, the lateral pressure may compress the bubbles, thereby deforming the reinforcement resin coating, by which the glass optical fiber therein may be bent minutely, so that the optical fiber may increase its transmission loss. Therefore, it is desired that no bubbles exist within the reinforcement resin coating.
Since the upper die 125a and lower die 125b are made of silica glass, the clamping pressure at the time of clamping cannot be made as high as that in the case of dies made of a metal. Consequently, a slight gap may occur between the parting faces 126a, 126b upon clamping as well. The UV-curable resin injected into the cavity 127 may seep into the gap and cure as it is, thereby forming a hardened fin-like object extending in a direction perpendicular to the surface of the reinforcement resin coating. The fin-like object attached to the surface of the reinforcement resin coating becomes an obstacle when carrying out an operation for further providing an outer coating on the connected optical fibers and the like, whereby it is necessary to shave off the fin-like object. Though the fin-like object is usually shaved off with a razor, it is necessary that the operation be carried out carefully, which takes considerable time and labor.